Marijuana plants flourish under the lights at a grow house in Denver. / Ed Andrieski, AP

by Donna Leinwand Leger, USA TODAY

by Donna Leinwand Leger, USA TODAY

As states increasingly adopt laws allowing medical marijuana, fewer teens see occasional marijuana use as harmful, the largest national survey of youth drug use has found.

Nearly 80% of high school seniors don't consider occasional marijuana use harmful - the highest rate since 1983 - and one in 15 smoke nearly every day, according to the annual survey of eighth-, 10th- and 12th-graders made public Wednesday.

More than one in five high school seniors said they smoked marijuana in the month before the survey, and 36% smoked marijuana during the previous year, according to Monitoring the Future survey of 45,449 students from 395 public and private schools. After four straight years of increasing marijuana use among teens, annual use among 10th and 12th graders stabilized and use by eighth graders declined slightly since 2010.

The survey has measured drug, alcohol and cigarette use since 1975.

"Whether this is more than a pause in the ongoing increase that we have seen in teen marijuana use in recent years is unclear at this point," the study principal investigator Lloyd Johnston said. Teens' growing belief that marijuana is not harmful suggests that marijuana smoking will increase, he said.

The growing number of state laws that allow marijuana for medical use contributes to teen perceptions that marijuana is not a harmful drug, said Dr. Nora Volkow, director of the National Institutes of Health's National Institute on Drug Abuse, which sponsors the study.

When teens perceive drugs as safe, drug use generally increases, Volkow said. Among eighth-graders, more than 50% don't see the harm of occasional marijuana use while 42% consider occasional use of marijuana harmful -- the lowest rate since the survey began tracking risk perception for this age group in 1991.

A study published this year in the Proceedings of the National Academy of Sciences found that heavy marijuana use beginning as a teen and stretching into adulthood causes an average drop of 8 points in IQ scores.

"That's a very robust indication that (smoking marijuana) may have long-term effects," Volkow said.

The 2012 survey found 6.5% of high school seniors smoke marijuana daily, up from 5.1% five years ago. Almost 23% smoke marijuana regularly. Among 10th-graders, 3.5% smoke marijuana daily, the survey found.

Among 12th-graders, 11% said they had used synthetic marijuana, known as K2 or Spice - about the same as last year, the first year the survey asked about it. Aside from alcohol and tobacco, synthetic marijuana is the second-most-widely used drug among 10th- and 12th-graders after marijuana. The federal government recently banned the drugs.

Marijuana use escalates dramatically after eighth grade, when 1.1% of the students report daily use.

"Marijuana use among teens remains at unacceptable levels," White House Office of National Drug Control Policy director Gil Kerlikowske said.

Most eighth-graders don't see the harm of occasional use, the survey found.

"I think that's the bad news in the survey -- the significant increases in the regular use of marijuana," Volkow said. "It's not just the occasional use. You have a very high rate of daily use. That's really a huge number."

Ethan Nadelmann, executive director of the Drug Policy Alliance, which advocates for decriminalization of drug use, says teens' perceptions of harm from marijuana are becoming more consistent with science.

"Kids know the dangers of cigarettes. They have a growing wariness about prescription drugs. They are aware that daily marijuana use is a very bad idea," Nadelmann said. "But they are also aware that occasional use is not much much problem."

Use of other illegal drugs continued to show a slow but steady decline. Past-year use of all illegal drugs except for marijuana is at its lowest point since 1997, the survey found.

"These long-term declines in youth drug use in America are proof that positive social change is possible," Kerlikowske said.

The prescription stimulant Adderall showed some signs of increasing abuse among 12th-graders this year, the survey found. Abuse of other prescription drugs, such as prescription painkillers like OxyContin and Vicodin, declined overall.

The survey for the first time measured use of an emerging stimulant drug known as "bath salts" and found low use among teenagers. Among 12th-graders, 1.3% said they had used the drugs, which can often be purchased on the internet or in drug paraphernalia stores.